1. Field of the Invention
This invention is in the field of etching integrated semiconductor circuits by the reactive ion etching technique using an improved gas mixture as the etchant.
2. Description of the Prior Art
Metal silicides are enjoying increased significance in the manufacture of VLSI (very large scale integration) circuits utilizing MOS (metal-oxide-semiconductors). One of the applications for these circuits is that of a low resistance conducting track and gate material in a polysilicon gate process. The polysilicon is not generally replaced by the silicide completely but is rather augmented by it in such a way that the silicide is applied over a doped polysilicon layer. The creation of fine structures in such double layers is a complex etching problem since a series of boundary conditions must be taken into consideration in the manufacture of these circuits. These boundary conditions are:
(a) high selectivity to SiO.sub.2, or to corresponding insulating layers since the ratio of layer thickness of polysilicide to SiO.sub.2 can be up to 50:1, with very thin SiO.sub.2 layers under the polysilicide; PA1 (b) anisotropy of the etching with perpendicular edges, which is particularly important in VLSI circuits; PA1 (c) employment of a photoresist as an etching mask, which frequently presents problems when using certain etching gases such as carbon tetrachloride; PA1 (d) providing the possibility of buried contacts; PA1 (e) achieving a good etching uniformity which is particularly important because of the so-called short channel effect; and PA1 (f) controlled etching times, so that high throughput is possible while the process is still subject to close quality control.
It is known from the prior art to etch structures containing polysilicides such as molybdenum and tungsten silicides. We know of only one brief suggestion regarding the etching of tantalum silicide structures which have considerable advantages over molybdenum and tungsten silicide structures because they have better temperature stability at high temperatures and good adhesion properties on polysilicon. This reference is in J.Vac.Sci.Technol. 17 (4) July/August 1980, pages 787-788. This reference suggests the etching of titanium, tantalum, molybdenum, and tungsten silicides in a plasma with a carbon tetrafluoride/oxygen mixture. The etchings are performed in part in a tunnel reactor and in part in a parallel plate reactor utilizing anodic coupling (see, in addition, J.Vac.Sci.Technol. 18 (2), March 1981, page 346). In principle, these layers can also be etched in the wet state but only with the loss of dimensions characteristic of wet-etching. It is characteristic of double layers that a multitude of edge shapes can appear during structuring, with only a few of the edge shapes being usable in circuit manufacture.
A number of methods for reactive ion etching of polysilicon are found in the European patent application 0 015 403 which suggests the utilization of gas mixtures consisting of sulfur hexafluoride (SF.sub.6), chlorine (Cl.sub.2), and an inert gas. With these methods, silicon is selectively etched whereby very good selectivity is achieved in the presence of SiO.sub.2 and silicon nitride. In addition, a directional etching can also be carried out wherein the substrates to be etched are laid on the HF-carrying electrode, so that the generated depressions have perpendicular lateral walls and the etching mask does not project over the edge of the etched depressions.
Still another method of the type previously discussed is known for molybdenum etching from the Japanese Journal of Appl. Physics, Vol. 21, No. 1, January 1982, pages 168-172. In this method, a gas mixture of carbon tetrachloride (CCl.sub.4) and oxygen is employed as the etching gas and a parallel plate reactor is operated with cathodic coupling.